Along with plot, I am always thinking about structure. Sometimes the story guides you to the best structure for its telling. Using letters seemed like the best way to tell this story. When writing these letters, the characters are selecting what they want to tell. In this case, the woman is writing in a way that would not endanger her or her family if her letters were found by the military authorities who took over the country, and the man is writing with the urgency of someone who could die at any minute while at sea.